Interpretive Summary: Drainage ditches are a common practice in humid climates, such as the Midwestern United States, to remove excess water from fields in order to improve crop yields and trafficability. These ditches are commonly dredged to remove buildup of sediments. Sediments can remove nutrients such as phosphorus from the water column, or contribute to elevated contaminant levels in the water column. It is unknown how ditch dredging impacts contaminant transport. This study was conducted to determine how dredging affects phosphorus transport in ditch water. Pre-dredged and dredged sediments were collected from a ditch that was being dredged, and placed in a flume that is used to simulate streams. Water with high levels of phosphorus was used to flow over the sediments, and water samples were collected periodically, followed by a period in which low phosphorus water flowed over the sediments. During the first portion of the experiment, the pre-dredged sediments removed phosphorus quicker than the dredged sediments, whereas in the second portion of the experiment, the phosphorus released from the sediments to the water column was quicker from the dredged sediments. At the end of both periods, the water flowing over the pre-dredged sediments contained the lower phosphorus concentrations than dredged sediments. These results indicate that while dredging is necessary to ensure adequate removal of water from agricultural fields, the process may increase the loss of nutrients from these watersheds. The impact of this research is that it has identified a common practice that may be detrimental to water quality, and methods can now be developed to reduce the potential negative impacts of dredging.

Technical Abstract:
Drainage ditches are a common practice in humid climates to remove excess water from fields to improve crop yields and trafficability. These ditches are commonly dredged to remove buildup of sediments thereby ensuring adequate removal of water from agricultural lands. Sediments can remove nutrients such as phosphorus from the water column, or contribute to elevated contaminant levels in the water column. It is unknown how the practice of ditch dredging impacts contaminant transport. The purpose of this study was to determine how dredging affects phosphorus transport in ditch water. Pre-dredged and dredged sediments were collected from a ditch that was being dredged, and placed in a fluvarium, which simulates a stream. Water with high levels of phosphorus (as high as 20 mg/L) was used to flow over the sediments, and water samples were collected periodically, followed by a period in which low phosphorus water flowing over the sediments. During the first portion of the experiment, the pre-dredged sediments removed phosphorus quicker than the dredged sediments, whereas in the second portion of the experiment, the phosphorus released from the sediments to the water column was quicker from the dredged sediments. At the end of both periods, the water flowing over the pre-dredged sediments contained the lower phosphorus concentrations compared to the dredged sediments. Final phosphorus concentrations at the end of the first phase were 1.3 mg/L for the pre-dredged sediment and 2.1 mg/L for the dredged sediment, while final concentrations for the second phase were 0.186 mg/L and 0.200 for pre-dredged and dredged, respectively. These results indicate that while dredging is necessary to ensure adequate removal of water from agricultural fields, the process may increase the loss of nutrients from these watersheds. The impact of this research is that it has identified a common practice that may be detrimental to water quality, and methods can now be developed to reduce the potential negative impacts of dredging.